The accumulation of dirt on engineered surfaces designed to function in outdoor environments can be an issue, for example on traffic signs, solar panels, and window glass, to name a few. Moreover, such surfaces frequently include surface coatings or applied polymeric films with specialized functions. For instance, traffic signs often utilize a retro-reflective surface that reflects light directly back to the source to enhance visibility to motorists. Solar panels often include a coating that decreases the reflection of light to enhance the harvesting of solar energy, and films might be applied to window glass to prevent fog formation or reflect specific wavelengths of light, such as infrared. The occasional condensation of water on such surfaces and the inevitable buildup of dirt can each compromise or even eliminate the functionality of the specialized coating. Furthermore, dirt accumulation has obvious deleterious effects on the aesthetics of many surfaces. In many cases, frequent cleaning to restore surface function is expensive or impractical. For at least these reasons, a coating or film that limits the condensation of water and the buildup of dirt on outdoor surfaces is highly desirable. Such coatings are also preferably durable (e.g., resist scratching and abrasion) and weatherable (e.g., resists degradation from moisture, UV light, temperature fluctuations, and combinations thereof) to function effectively in outdoor environments for acceptable periods of time.
Acid-sintered nanosilica coatings have been prepared by acidifying an aqueous colloidal suspension of nanometer-scale particles to a pH of less than 5, for example 2-3. Such silica nanoparticle coatings include those, for example, described in co-owned U.S. Patent Application Publication Nos. 2011/0033694 (Jing et al.) and 2010/0035039 (Jing et al.) and in co-owned International Application Publication Nos. WO 2012/064646 (Riddle et al.), WO 2011/139573 (Pett et al.), and WO 2012/047422 (Hebrink et al.). At a pH of 2-3, the particles possess little or no net charge, and can thus aggregate and fuse. Upon drying, the particles sinter at temperatures much lower than would be required without acidification. The result is a hard, porous, and hydrophilic coating that can be applied to various substrates. Dry dirt and dust exhibits reduced adhesion to nanosilica-coated surfaces relative to common glass or polymeric substrates. This slows the buildup of dirt in outdoor environments and reduces the need for cleaning.
Due to their hydrophilicity, acid-sintered nanosilica coatings can also prevent fogging. Fog forms when water condenses from saturated vapor and beads up into micron-sized droplets that scatter light. Highly hydrophilic surfaces can resist fogging because any water condensing on the surface forms a thin film that is transparent. This anti-fogging property makes nanosilica coatings even more desirable for outdoor applications as fogging causes many of the same deleterious effects on engineered surfaces as does soiling. While acid-sintered nanosilica coatings have been shown to be acceptably stable to moisture and UV light, they do not possess sufficient abrasion resistance for some applications.